Retrofit tailtank-VentusB?

My B has a addon tank which holds 6 liters, it *appears* that it was ins=
talled by glassing it in the cavity low in the tail, and I have been told=
it is a stock Schemp tank, fills from a small orifice on top of the tail=
No cutting open of tail apparently. I have a friend who would like to=
do this mod to his ship...anyone out there who has done this mod or is f=
amiliar with it?